net/mlx5: Fix create autogroup prev initializer

The autogroups list is a list of non overlapping group boundaries
sorted by their start index. If the autogroups list wasn't empty
and an empty group slot was found at the start of the list,
the new group was added to the end of the list instead of the
beginning, as the prev initializer was incorrect.
When this was repeated, it caused multiple groups to have
overlapping boundaries.

Fixed that by correctly initializing the prev pointer to the
start of the list.

Fixes: eccec8da ('net/mlx5: Keep autogroups list ordered')
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>